cancellation of contract not done

I called to cancel my tracker account on the 7 March. Nothing was done. Called again in April to confirm cancellation, was told cancellation still not done and they will get someone to call me. NO ONE CALLED! Then I get an sms to say that my account is in arrears. Called today to find out whats going on. The most unhelpful woman in cancellation dept tells me that I am liable for the arrears amount even though I called to cancel in March. So now I have to pay for April and May for a service I never wanted.